Method for spiralling electrical resistors

ABSTRACT

A method for manufacturing electrical resistors of the type comprising turns of conductive metal or alloy film formed on a cylindrical substrate with a spiral track free of the film defined between the turns. The substrate is first provided with a complete film, and the tip of a conductive stylus is then brought into engagement with the film adjacent one end of the substrate. Current is passed through the stylus whereby the film is heated in the area engaged by the tip and film vaporization occurs. Relative movement between the substrate and stylus will result in a continuous spiral track free of the film as a result of the vaporization. The changes in resistance which occur during the processing can be continuously measured so that the processing can be discontinued when a desired resistance value is reached.

United States Patent Martin et al.

[451 July 11, 1972 of London, Ontario; John R. Lemon, East London, Ontario, all of Canada [73] Assignee: Welwyn Canada Limited, London. On-

tario, Canada [22] Filed: May 13, I970 21 1 Appl. No.: 36,965

3,534,472 l0/l970 3.539.309 ll/l970 OTHER PUBLICATIONS IBM Technical Disclosure Bulletin, Pulsed High-Voltage Resistor Trimmer." Vol. ID, No. 4. September I967 Primary Examiner-John F. Campbell Assistant Examiner-Victor A. DiPalma Attorney-McDougall, Hersh & Scott ABSTRACT A method for manufacturing electrical resistors of the type comprising turns of conductive metal or alloy film formed on a cylindrical substrate with a spiral track free of the film defined between the turns. The substrate is first provided with a complete film, and the tip of a conductive stylus is then brought into engagement with the film adjacent one end of the substrate. Cun'ent is passed through the stylus whereby the film is heated in the area engaged by the tip and film vaporization occurs. Relative movement between the substrate and stylus will result in a continuous spiral track free of the film as a result of the vaporization. The changes in resistance which occur during the processing can be continuously measured so that the pmcesing can be discontinued when a desired resistance value is reached.

IOClalms, 2 Drawing Figures PKTENTEDJIJLH m2 3.675.317

FIG. 1 k W 0 M 1 22 FIG. 2

IN VENTODS JohnPezer-Mar iorl DamlUohn Bemrzger METHOD FOR SPIRALLING ELECTRICAL RESISTORS This invention relates to a method for manufacturing electrical resistors. The invention is particularly concerned with resistors comprising spiral turns of conductive metal or alloy films located on an insulating substrate.

Electrical resistors, having cylindrical bodies of insulating materials and employing deposited films as the resistive element on the surface of the cylindrical body, are adjusted to the desired ohmic value by cutting a helical track in the film along the longitudinal axis of the cylindrical body. The operation of cutting the helical track in the film is known as spiralling the resistor. The spiralling accomplishes two objectives. First, it increases the over-all resistance of the resistor by factors ranging from one to several hundred, resulting in a resistive gain 6 according to the formula:

d v 1 b (rr+b) where R l is the resistance of the spiralled resistor R is the resistance of the pre-spiralled resistor, and

a, b and d are, respectively, the spiral track width, the spiral film width, and the substrate diameter. (see FIG. 1)

Second, it provides the means for the adjustment of resistance of the resistor to a predetermined value, within a predetermined degree of accuracy, since the length of the track can be varied.

Methods of selective film removal from cylindrical substrates involved in the spiralling of metal or alloy film resistors are conventionally mechanical or thermal. Mechanical modes of spiralling involve the use of abrasive wheels rotating at high speed or the use of a fine stream of abrasive powders sprayed at high speed. The wheel or the spray cuts a spiral track on the cylindrical surfaces of the resistor by means of rotating the resistor along its long axis and advancing the resistor body, or the cutting mechanism, along the long axis. The ratio of the speed of rotation and the speed of advance, determines the pitch of the spiral and consequently the number of spiral turns on the resistor.

Mechanical modes of spiralling remove the resistive film by cutting a groove through the film into the substrate. The width of the groove and the depth depend on the mechanical factors involved and vary with the cutting profile of the wheel, the size of the abrasive particles, etc. The cutting action during spiralling chips the substrate to a certain extent and results in a groove with rough or pitted edges. Consequently, the edges of the film forming the spiral ribbon of the resistor become ragged. Typical dimensions for spiral track width 0, and edge roughness of the resistive film, for mechanically spiralled resistors range from 0.020 to 0.050 inch and from 0.005 to 0.020 inch, respectively. The edge roughness represents an intrusion into the resistive film at both sides of the ribbon, and it decreases the effective width b. This limits the useful width of the resistive ribbon for electrical conduction and sets a lower limit to the over-all width b below which an open circuit results. In practice, the ratio b/a is kept above the minimum value of 2. The resulting maximum resistive gain, for resistors, spiralled by these methods, is expressed by the formula:

I'l'ldX For example, the maximum resistive gain for a resistor, with body diameter of 0.250 inch (and of any length) and spiral track width of 0.020 inch, is 250, and that of a resistor, with body diameter of 0.100 inch and the same 0.020 inch track width, is 40.

Thermal modes of film removal during resistor spiralling are an improvement over the mechanical methods in certain respects. They involve the use of a C0, laser, a pulsed ruby and other lasers, or the use of energetic beams of electrons or ions. The beams (either light or electrons, etc.) are usually monochromatic, therefore, they can be finely focused onto the metal or alloy film on the cylindrical substrate of theresistor. Spot sizes of a few, to a few tens of micrometers, are typical. The energy concentrated on the small spot from the energetic lasers, etc., is sufficient to heat the film to a high enough temperature for evaporation to occur. The resistor is again rotated and advanced during spiralling as in the mechanical method described above and a spiral track results.

Problems encountered with thermal evaporation spiralling are quite serious. High energy beam sources are rather expensive and require elaborate installation. The energy deposited on the resistor, to evaporate a spiral track in the film, is sufficiently high to effect the substrate and to cause thermal damage to the edges of the resistive film ribbon remaining on the substrate. Cracking of the substrate or buckling of the film may occur and the film may oxidize due to the high temperatures involved. Some of the problems may be remedied by extremely short pulses of the cutting beam, protective inert atmosphere and other methods but they add to the expense of the operation. Due mainly to the detrimental thermal effects in the resistive ribbon, dimension b is restricted to widths such, that at least the central 50 percent of the film should be unaffected by the high cutting temperature. In practice, this results in resistive gains equal to, or slightly higher than those typical for mechanical spiralling.

Resistive metal and alloy films range in thickness from a few hundred to a few thousand angstrom units with surface resistivities of from a few tenths to a few thousand ohms per square. These films possess metallic characteristics, i.e., have close to zero voltage coefficient of resistivity (VCR) and slightly positive temperature coefficient of resistivity (TCR), low current noise, etc. Using mechanical or thermal spiralling methods to fabricate metal film resistors with metallic characteristics results in a practical upper limit of resistance values for each resistor size. Assuming that a film with 2,000 ohms per square resistivity has metallic characteristics (and this is about the practical upper limit for normal metals and alloys) and it is used on a substrate with Her d 2, i.e., with an un' spiralled terminal resistance of 4,000 ohms, then the maximum resistance value attainable on a normal 0.250 inch diameter body after spiralling is l megohm, and that on a normal 0.100 inch diameter body is 160,000 ohms. Attempts to fabricate resistors with higher values require films with higher resistivity values. This in turn requires thinner films. However, such films have negative TCR and large VCR. They are also noisy, are easily affected by their environment, and are thermally unstable.

It is a general object of this invention to provide an improved method for spiralling electrical resistors.

It is a more specific object of this invention to provide a new method of non-mechanical spiralling of film resistors. producing ultra-high gains of up to 25,000 with minimum unevenness at the edges of the tracks, which involves minimal mechanical damage to the film and which eliminates damage to the resistor body, and which is capable of producing metal or alloy film resistors to resistive values up to megohms with metallic conduction behavior, i.e., metallic TCR, no VCR, and improved thermal, environmental and load performance.

These and other objects of this invention will appear hereinafter and for purposes of illustration, but not of limitation, specific embodiments of the invention are shown in the accompanying drawings in which:

FIG. I is an elevational view of a resistor adapted to be manufactured in accordance with the concepts of this invention; and,

FIG. 2 is a circuit arrangement adapted to be employed in the practice of the invention.

The present invention involves a new method of spiralling, capable of producing gains of up to 100 times those attained by conventional methods. With this new method, metal and alloy film resistors, utilizing good stable films that possess metallic characteristics, can be fabricated with resistance values of up to 100 megohms. The method combines the inexpensive features of mechanical spiralling, with the fine dimensional features of thermal spiralling, without many of the detrimental side effects of both.

The spiral track is created by the selective removal of the metal or alloy film from the substrate. This is accomplished by utilizing Joule heating from electrical current, with the current passing through the film and a small metal stylus in contact with the film. The tip radius of the stylus in contact with the film is typically 0.001 inch and the force pressing the tip to the film is a few tenths of 1 gram. The resulting contact area between film and stylus is a few tens of square microns. A few milliamperes of current passed through the circuit represents a very high current density at the point of contact. This will cause Joule heating at the point and its immediate neighborhood, and evaporate the film at the point of contact, leaving a circular hole in the film of about 0.001 inch in diameter. As soon as evaporation occurs, the electrical circuit is broken and nothing further happens. lf the resistor is now rotated under the stylus, the edge of the film around the circular "hole" will come in contact with the tip of the stylus, the current will start flowing again, the film at the contact will evaporate, and another hole in the film will be created. The new hole is semicircular or crescent (half-moon) shaped. The process continues, as the resistor is further rotated, resulting in a continuous track in the film. The track is about 0.001 inch wide, being composed of successive semi-circular areas of 0.00] inch diameter with an edge roughness of approximately 0.0002 inch. if the rotating resistor is new advanced along its long axis under the stationary stylus, the resistor is spiralled.

The mechanism necessary to spin and advance resistors being spiralled by the present method is far simpler than conventional mechanical spiralling machines, due mainly to the substitution of the heavy and cumbersome spiralling wheel or nozzle by the minute stylus. A further simplification in machinery is inherent with the present system; the spiralling stylus need not be lifted off the film instantaneously when the desired terminal resistance of the resistor is reached, unlike the mechanical spiralling wheel or the nozzle. To stop cutting the track, it is sufficient to switch off the cutting current at the source in the instant the desired resistance value is reached. Without the current flowing through the contact area, the stylus is unable to mark the film due to the very slight mechanical force between it and the film. The small stylus pressure also guarantees that the substrate is left undamaged during spiralling.

The current source for electrical stylus spiralling may be DC, AC or pulsed DC depending upon the rate at which the film is cut, the film thickness, and the film resistivity. The current source is a high impedance constant current power supply in order to minimize the variation of current due to changes in circuit resistance. The cutting current is usually a few milliamperes.

The duration of the current in case of a DC source depends on the rate of advance of the stylus into the film, i.e., the angular speed of the rotating resistor. At zero speed the current passes for a few micro-seconds before the circuit is broken due to metal evaporation. During these few microseconds, the energy deposited in the film under the tip of the stylus from the current evaporates the metal in the immediate area and the energy is withdrawn from the film by the evaporating metal. As energy deposition stops almost immediately with break in the circuit, there is no thennal damage to the film lying farther than about 0.0002 inch from the edge of the evaporated track. This process is in sharp contrast with the laser or other beam evaporation processes in which the energy deposited on the resistor is absorbed partly by the film and partly by the substrate, therefore, larger doses of energies are required to evaporate the film. This results, naturally, in gross heating of the resistor and consequent possible damage to film and substrate.

At non-zero stylus speed, the duration of the current is increased as the stylus moves into the film. 1n the extreme case, at high stylus speed, the rate of stylus advance into the film may be higher than the rate of evaporation of the film and the current is, therefore, sustained. Under this condition, evaporation requires a larger energy input into the film, consequently the remaining resistive ribbon may be adversely affected by the resulting heat. To overcome this, an AC or pulsed DC source is used in practice. A pulsed source is generally preferred as it provides for the adjustment of the amount of energy and the pulse length independently. These two parameters are included in the variables of the present invention, along with the tip radius of the stylus and the angular speed of the resistor for spiralling metal and alloy film resistors. With the proper adjustment of these four spiralling variables, films ranging in resistivities from a fraction, to thousands of ohms per square, can be spiralled.

The above features of electrical stylus spiralling permit a minimum resistive ribbon width b of 0.004 0.005 inch. Substituting this and an 0.001 inch track width a into formula l). gains of about 25,000 and about 4,000 may be obtained for re sistors with diameters of 0.250 and 0. 100, respectively. This is a hundredfold improvement over gain figures obtainable with mechanical spiralling. Consequently, the electrical stylus spiralling is capable of producing metal and alloy film resistors to maximum values of megohms and I6 megohms, respectively, on the larger and smaller substrates cited above.

In specific applications of the invention, cylindrical glass, ceramic or plastic bodies such as shown at 10 in the drawings, having a diameter between 0.050 inch and 1.000 inch, and a length between 0. 100 inch and 10 inches have been employed. Any suitable metal or alloy film 12 having a resistivity between 0.] ohm per square and 500,000 ohms per square is deposited on the substrate surface. The film is terminated at the two ends of the substrate and the substrate is placed in a device capable of rotating it around its long axis. The rotating means preferably engage conductive blocks 14 which act as terminals for the system. A stylus 16, made of a suitable metal or alloy, such as tungsten or carbon-steel, having a tip 18 defining a variable tip radius, typically 0.001 inch, is brought into contact with the resistive film on the resistor with a force between the tip of the stylus and the film between 0. l and 10 grams.

A power supply 20, DC, AC or pulsed DC with variable source impedance and voltage, and variable frequency or duty cycle and pulse width, is connected between the stylus and the film as shown in FIG. 2. A resistance measuring device 22 and a feedback device 24 are also included in the circuit as shown in FIG. 2. The resistor is rotated under the stationary stylus, and the resistor or the stylus is moved in the axial direction of the resistor, the stylus thereby drawing a spiral on the surface of the resistor.

In operation, the power is switched on, and the stylus begins to cut the track 26. The process continues until the resistor reaches a predetermined resistance value, as measured by the device 22. At this instant, a signal through the feedback circuit 24 switches ofi the power supply 20 and the cutting ceases. The stylus is lifted off the resistor and the machine is stopped.

The circuit arrangement described may be any conventional type which will accomplish the described function. In a typical arrangement, the feedback device 24 comprises an amplifier whose input is the output of a resistance bridge 22 with the output of the amplifier operating to actuate a relay in the power supply 20. As the resistor is spiralled, its resistance increases and approaches the value pre-set on the measuring bridge. When the resistor reaches the pre-set value, the bridge output decreases to zero volts, and then a change in polarity occurs. The change in polarity of the input to the amplifier then triggers the relay in the power supply to turn off the spiralling current.

The control of current through the stylus and film may be accomplished at the source of the current, i.e., at the power supply, or by the speed of rotation of the resistor under the stylus, i.e., the speed of advance of the film being cut relative to the stylus. The former may be controlled by the type of current generated by the supply, DC, AC or pulsed current. In the case of DC and stationary stylus, current flows until the circuit is opened due to film evaporation. The duration of current flow is controlled by the current level, i.e., at high currents evaporation occurs faster than at low currents.

In the case of a moving film with DC supply, the same mechanism is responsible for film evaporation as in the stationary case, provided the speed of film relative to stylus is slow. In this case the sequence is as follows: current flows, film evaporates, resistor moves constantly, new contact is made at edge of evaporated region, current flows, new area evaporates, new contact is made, etc. The current is on for a while and off for a while.

If the film is moved faster, the off time" will decrease since the stylus will come to the new film edge sooner. The on time will remain the same if the current level remains the same. An increase in speed will decrease the off time to zero, i.e., the current will be continuous and a track will still be evaporated. A further increase in speed will make the stylus skip" and evaporation of the track will not occur or uneven evaporation will result. As it is desirable to spiral resistors at high speed. the highest speed possible which still avoids skipping is desired. This may be accomplished by increasing the level of current to decrease the on time for current flow, but in this case the evaporated circular area under the stylus becomes large. and consequently the spiral track becomes wide.

The foregoing also generally applies to AC current. Pulsed DC current however offers advantages over DC and AC. if high speed is desired, the "on time" and off time" for current flow can be adjusted by the operator on the power supply by controlling the pulse width, duty cycle (ration of on to off times) and the frequency of the pulses (rectangular waves), independent of the natural DC on and off times. Moreover, the pulse height (level of current) can also be controlled and thus the electrical energy per unit time delivered to the film can be portioned in the right manner for minimum track width and maximum cutting speed.

In a typical example, a nickel alloy film is deposited on a cylindrical Pyrex glass substrate, the film having a thickness of 500A. and the substrate being rotated under a conductive stylus with 000i inch radius resting on the nickel alloy film, the relative speed of stylus and film being 0.4 inch per second. Current is made to flow through the stylus and the film, supplied by a pulse generator, in pulsed mode, with frequency of 8,000 cycles per second, pulse width of 2 microseconds and pulse height ofl milliamperes.

With a tip radius of 0.001 inches, and with a contact pressure in the order of 0.5 grams, the evaporated area will have a diameter or width of approximately 0.001 inches. The tip of the stylus actually will have point Contact with the film, however, the film evaporates from under the tip as well as in the immediate surrounding area. The width of the evaporated area can be controlled by varying the current; however, a track width of 0.001 inch provides a highly practical value.

The same basic procedure can be used for manufacturing a large number of resistors, for example 20, simultaneously. The resistors may be inserted in a spiralling machine in side-byside relationship by means of separate chucks having common drive means. A separate stylus and circuit is utilized for each resistor so that the spiralling current to a particular resistor can be independently cut off and track cutting discontinued, even though rotation of the complete assembly continues. The spiralling currents for the individual resistors will be cut off one-by-one, depending upon the resistance measuring means employed.

The method of the instant invention provides many improvements in metal and alloy film resistors. The narrow spiral tracks which are achieved result in high resistive gain in the order of l00 times higher than can be achieved with conventional techniques, the invention thus extending the limit of resistance that can be achieved with metal film resistors by one hundred times. Spiral tracks as small as 0.001 inch and with edge roughness of only 0.0002 inch result from the manufacturing techniques described. The substrate damage during spiralling is effectively eliminated, and there are no significant amounts of film residue in the spiral tracks formed. The resistive film ribbon adjacent the spiral tracks is not heated during spiralling and, therefore, unwanted stresses are avoided. For this same reason, the TCR, VCR and stability of the film remains unchanged.

The products of the invention are characterized by low current noise since there is minimal edge damage. Since there are no heavy machinery components, relatively simple and inexpensive equipment can be employed. In spite of the relatively simple nature of the equipment, including the electrical circuitry, greater accuracies in resistance value are obtained due to the instant cessation of film cutting activity when a desired resistance level is achieved. Finally, the system lends itself to spiralling of a plurality of resistors simultaneously, again without the need for expensive or complex equipment,

It will be understood that various changes and modifications may be made in the above described construction which provide the characteristics of this invention without departing from the spirit thereof.

That which is claimed is:

l. A method for manufacturing electrical resistors of the type defining an elongated film of metal or alloy material formed in spiral fashion on a cylindrical substrate with a spiral track free of said material defined between turns of the film, said method comprising the steps of providing means for supporting said substrate, providing an electrically conductive stylus having a tip diameter substantially equal to the desired width of said spiral track, locating said tip in contact with said film, connecting said film and stylus in an electrical circuit, passing current through said stylus and film whereby the film portion in contact with said tip is heated, continuing current flow until said film portion vaporizes, said tip being maintained relative to said film portion until said vaporization is completed with the elimination of said film portion resulting in breaking of the electrical circuit, providing means for achieving relative movement between the film and stylus to thereby move the stylus tip into engagement with an adjacent portion of film, said relative movement to an adjacent film portion occurring each time said circuit is broken, and continuously repeating the current flow, film evaporation, and relative movement to achieve formation of said spiral track.

2. A method in accordance with claim 1 including the step of moving said support whereby said substrate is rotated about its axis and is simultaneously moved longitudinally relative to its axis, said stylus being held stationary during movement of the substrate.

3. A method in accordance with claim 1 including the step of attaching resistance measuring means between terminals located in opposite ends of the film whereby changes in re sistance can be measured during formation of the spiral track.

4. A method in accordance with claim 4 comprising the steps of connecting a feedback circuit to said resistance measuring means, and setting said feedback circuit to deliver an electrical signal to the means employed for passing current through said stylus and film, said signal operating to prevent further operation of said current passing means when the resistance measured by said resistance measuring means reaches a predetermined value.

5. A method in accordance with claim 1 wherein said stylus defines a tip radius of about 0.001 inch.

6. A method in accordance with claim 1 wherein said electrical circuit supplies DC current for passage through said stylus and film.

7. A method in accordance with claim 6 wherein said current is pulsed DC current.

8. A method in accordance with claim 1 wherein said electrical circuit supplies AC current for passage through said stylus and film.

9. A method in accordance with claim 4 wherein a plurality of separate supports are provided for simultaneously manufacturing a plurality of electrical resistors, a common drive means for said supports, and individual current supply means for each resistor, said feedback circuit operating to individually cut olf said current supply means as each resistor reaches a desired resistance level.

10. A method in accordance with claim 1 wherein said tip is held stationary during vaporization of the UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 6 75 317 D t d July 11 1972 Inventor(s) John P. Marten, et. a1

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

On the cover sheet, item [72], "John P. Martin" should read John P. Marton Signed and sealed this 2nd day of January 1973.

(SEAL) Attest:

EDWARD M.FLETCHER,JR.

ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents ORM PO-OSO (10-69) USCOMM-DC U576-P59 U 5 GOVERNMENT PRINTING OFFICE; IQ. D-lli-JSJ. 

1. A method for manufacturing electrical resistors of the type defining an elongated film of metal or alloy material formed in spiral fashion on a cylindrical substrate with a spiral track free of said material defined between turns of the film, said method comprising the steps of providing means for supporting said substrate, providing an electrically conductive stylus having a tip diameter substantially equal to the desired width of said spiral track, locating said tip in contact with said film, connecting said film and stylus in an electrical circuit, passing current through said stylus and film whereby the film portion in contact with said tip is heated, continuing current flow until said film portion vaporizes, said tip being maintained relative to said film portion until said vaporization is completed with the elimination of said film portion resulting in breaking of the electrical circuit, providing means for achieving relative movement between the film and stylus to thereby move the stylus tip into engagement with an adjacent portion of film, said relative movement to an adjacent film portion occurring each time said circuit is broken, and continuously repeating the current flow, film evaporation, and relative movement to achieve formation of said spiral track.
 2. A method in accordance with claim 1 including the step of moving said support whereby said substrate is rotated about its axis and is simultaneously moved longitudinally relative to its axis, said stylus being held stationary during movement of the substrate.
 3. A method in accordance with claim 1 including the step of attaching resistance measuring means between terminals located in opposite ends of the film whereby changes in resistance can be measured during formation of the spiral track.
 4. A method in accordance with claim 4 comprising the steps of connecting a feedback circuit to said resistance measuring means, and setting said feedback circuit to deliver an electrical signal to the means employed for passing current through said stylus and film, said signal operating to prevent further operation of said current passing means when the resistance measured by said resistance measuring means reaches a predetermined value.
 5. A method in accordance with claim 1 wherein said stylus defines a tip radius of about 0.001 inch.
 6. A method in accordance with claim 1 wherein said electrical circuit supplies DC current for passage through said stylus and film.
 7. A method in accordance with claim 6 wherein said current is pulsed DC current.
 8. A method in accordance with claim 1 wherein said electrical circuit supplies AC current for passage through said stylus and film.
 9. A method in accordance with claim 4 wherein a plurality of separate supports are provided for simultaneously manufacturing a plurality of electrical resistors, a common drive means for said supports, and individual current supply means for each resistor, said feedback circuit operating to individually cut off said current supply means as each resistor reaches a desired resistance level.
 10. A method in accordance with claim 1 wherein said tip is held stationary during vaporization of the film. 